One by one, Mohammad and Munadi thread scallop shells on to thin metal rods, breaking the monotony with quiet chatter in their native Javanese. The shells will soon be used to cultivate oysters, a speciality in this region of western Japan.

Neither of the men, crouching on the floor of a shed overlooking Japan’s Inland Sea, had even seen an oyster before they came to Akitsu, a tiny port town in eastern Hiroshima prefecture, in April this year.

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They are part of a growing foreign workforce that policymakers see as a solution to Japan’s shrinking, ageing population and a stubbornly low birthrate.

Last week, the administration of prime minister Shinzo Abe approved legislation that will open the door to as many as half a million foreign workers by 2025, in what some are calling the end to Japan’s traditional opposition to large-scale immigration. The bill is expected to pass by the end of the year and go into effect next April.

Japan – one of the world’s most homogenous societies – has long resisted foreign labour, with exceptions made for those in professions such as teaching, medicine, engineering and the law. Mohammad and Munadi are part of a government-run foreign technical trainee programme that is supposed to provide workers from developing countries with skills they can take back to their home countries after five years.

Critics say employers abuse the scheme for cheap labour, with many failing to pay proper salaries and forcing interns to work long hours. In addition, the programme, which employed just over 260,000 foreign workers last year, does not include enough people with the specific skills required in sectors of the economy that are suffering from a labour shortage.

There were 1.28 million foreign workers among Japan’s workforce of 66 million in 2017 – double the number in 2012. But many are university students or technical trainees who, like Mohammad and Munadi, are not permitted to stay indefinitely. Unemployment dropped to at just 2.3% in September and there are 163 job vacancies for every 100 job seekers – the highest job availability for more than 40 years.

‘Not a conventional immigration policy’

Under the new legislation, foreign workers will be divided into two categories. Those with skills in sectors experiencing labour shortages will be allowed to work for up to five years but cannot bring their families with them. Those with more advanced skills will be able to bring family members and renew their visas indefinitely, and may eventually apply for permanent residency. Members of both groups must pass a Japanese-language exam.

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“We are not pursuing a conventional immigration policy,” Abe told MPs, adding that most foreign workers would stay in Japan for limited periods and that the policy would be reviewed in the event of an economic downturn or easing of labour shortages in particular sectors. “It would be wrong to force our values on foreigners. Instead, it’s important to create an environment in which people can happily coexist.”

But some experts disagree. “I think this is a de facto shift to an immigration policy,” said Hidenori Sakanaka, the former head of the Tokyo immigration bureau.

The prospect of a significant rise in the number of immigrant workers prompted a backlash from opposition parties.

The rightwing Japan First party complained that an influx of foreign workers would place intolerable pressure on welfare services and lead to higher crime rates.

Yuichiro Tamaki, leader of the centrist Democratic Party for the People, voiced concern over pressure on wages and social services. But he became the first party leader to support a European-style immigration policy that, he said, should ensure equal pay for equal work and allow foreign workers to bring their families to Japan.

The current edition of the rightwing magazine Sapio features a series of articles warning of a rise in violence, sex crimes and cultural clashes, while the private broadcaster Fuji TV was criticised for a recent programme about visa overstayers that demonised immigrants.

Munadi, left, and Mohammad. Photograph: Justin McCurry for the Guardian

The public appears more tolerant, however. A survey by the TV Tokyo and the Nikkei business newspaper showed 54% of Japanese voters favoured allowing in more unskilled foreign workers, with 36% against. Support for the move was particularly high among younger people.

The liberal Asahi newspaper said Abe had failed to address “a slew of concerns about its hasty initiative to drastically increase the number of foreign workers”.

“Whether they are called immigrants or not, the government has a responsibility to lay out a viable and convincing vision of the future of Japanese society where foreign workers and Japanese citizens can live together in harmony and feel secure,” the newspaper said, adding that the change was “bound to have a far-reaching effect on Japanese society”.

Those changes are already being felt in Hiroshima prefecture’s fisheries, where one in six workers is foreign – the highest rate in any industry in Japan. Among fishermen in their 20s and 30s, the ratio is one in two.

‘Places like this can’t survive without foreign workers’

In Akitsu, young fisheries workers from overseas now outnumber their ageing Japanese counterparts 33 to 30.

Takatoshi Shiba, head of the Akitsu fishermen’s cooperative, jokes that at 67, he is relatively young compared to his Japanese colleagues. “It feels like a wasted opportunity because the trainees spend time learning the job and getting used to life here, and then they have to go home after a few years,” says Shiba. “I don’t think the government has any choice but to act soon. Places like this can’t survive without foreign workers.”

Mohammad and Munadi say they have adapted well to life in rural Japan, although neither has plans to stay more than three years. They spend their days off shopping in nearby Hiroshima and playing badminton, and can now buy halal meat from the local supermarket. In just a few months they have acquired enough conversational Japanese to communicate with their neighbours and other trainees from China, the Philippines, Malaysia and Vietnam.

“We get on well with our Japanese colleagues and bosses,” says Munadi, 27, who left Java in April just after his wife gave birth to their first child. “And we get paid a lot more here than we would back in Indonesia.”

Mohammad agrees. “The work is no problem, but we miss our families,” he says as he removes another scallop shell from the pile in front of him. “But we are happy here.”